Photovoltaic devices (or cells) are often manufactured by forming or depositing a light-absorbing layer, along with various other layers, onto a transparent (e.g., glass) substrate. Among the other layers formed are a front contact, typically formed between the substrate and the light absorbing layer, and a back contact, typically formed on a side of the light-absorbing layer opposite the substrate, and perhaps an electron reflector layer formed between the light-absorbing layer and the back contact. One possibility for the material used in the electron reflector layer is cadmium-manganese-telluride (CMT).
In order to maximize the performance of CMT for use in photovoltaic devices, it is generally considered to require doping (e.g., p-type doping). The ideal cation vs. anion composition, and the methods of achieving such, have yet to be established.
There are two opposing ideas regarding cation vs. anion composition depending on the method of doping. Doping may be introduced either by internal defects, in particular cadmium vacancies, or by external dopants, such as copper. In the former case, one seeks cadmium-poor conditions. In the latter, one might consider using cadmium-rich conditions. Based on conventional wisdom related to the use of cadmium telluride in photovoltaic devices, it is generally assumed that cadmium vacancies are desirable.